The present invention is directed to the implementation of a reliable data communications system using the horizontal scan lines of video signals, and is particularly concerned with the minimization of problems which are caused as a result of false acquisition of horizontal scan lines and missed or dropped lines.
In the transmission of digital data, for example from one computer to another, it is common to employ existing communication media where the data is to be transmitted over large distances, and thereby avoid the need for dedicated data transmission lines. For example, the use of telephone lines to communicate data between computers, via modems, is one popular form of data transmission. However, since telephone lines were originally designed for voice grade communications, they offer limited bandwidth capabilities for data communication. As a result, the speed at which data can be transmitted over telephone lines, i.e., the available baud rate, is limited.
In an effort to increase the speed at which data can be efficiently transmitted, other existing forms of transmission media are being evaluated. One such medium is the television communication network. The transmission bandwidth that is available in a standard television channel can readily accommodate the transmission of data at higher rates. Furthermore, the availability of television reception at almost any locale makes television transmission a viable medium for communicating data from a central location to another location at a relatively low cost.
Typically, data is transmitted in a television signal by inserting the data in one or more lines of the vertical blanking interval of a standard television field. Since the vertical blanking interval does not contain any video information, e.g., any information pertaining to the image to be reproduced on a television screen, the placement of data on one or more of these lines of the signal does not interfere with the normal television program.
The transmission of data via television signals is not without its attendant limitations, however. Among these are the various types of noise that can be encountered when the television signal is transmitted through a commonly employed transmission medium, e.g., over the air or through cable. The effect of this noise is to create errors in individual bits of the data. By encoding the transmitted data with a suitable error checking code, individual bit errors can be readily identified in the received data and corrected.
Systems which insert digital data onto a horizontal scan line of a video signal typically utilize a specific bit pattern as a preamble which is transmitted with the data and which identifies the accompanying data, to enable it to be acquired by a receiver. Many of these systems use specific bit patterns for the preamble portion of the transmitted data which have properties to help prevent missing data in the presence of noise. The specific bit patterns are typically chosen so that receivers can select the proper data in the presence of a limited number of bit errors in the received bit pattern. Systems which incorporate these features thus enable data to be reliably reconstructed at the receiving end without missed or dropped lines of data, even when noise results in bit errors.
However, such systems do not provide fail-safe transmission and reception of the data via the television signals. Because the system has a certain amount of tolerance in choosing the horizontal scan line or lines containing data which is to be processed, the false acquisition of data can, and often does, occur. It is possible to decrease the frequency of false acquisition by making the receiver more selective in its acceptance of data. However, such a change increases the possibility that desirable data would be missed in the presence of noise. Accordingly, it would be desirable to provide a system of data transmission via the use of television signals whereby data could be accurately transmitted and received without missed or dropped horizontal lines of data while at the same time minimizing the false acquisition of data.